Is the punishment for one who mocks people to be afflicted with what he mocked? And how does one repent from that? And is supplicating for the one who gloated over him sufficient to avoid being afflicted with what was mocked?

Rejoicing at others' misfortunes (shamata) and mocking them are forbidden and considered major sins. If this occurs only within oneself, without outward manifestation or indulgence, then there is no accountability for it. As for repentance from this sin, it is achieved through repenting from sins related to the rights of people. Al-Ghazali mentioned that repentance from backbiting, slander, or insult is achieved by disavowing oneself to those before whom one committed the act, and seeking forgiveness from the person involved if possible without causing greater discord; otherwise, one should turn to Allah and seek much forgiveness. Whatever can be done to appease the adversaries should be done, and whatever cannot, one should turn to Allah with humility and sincerity, asking Him to appease them. It is also necessary to increase good deeds and acts of obedience, for Allah Almighty says: "Indeed, good deeds do away with evil deeds," and the Prophet, peace and blessings be upon him, said: "Follow up a bad deed with a good deed, it will erase it."